1. Field of the Invention
This invention relates to a mismatched lattice semiconductor device, and more particularly a gallium nitride semiconductor device formed on a substrate.
2. Discussion of the Background
A bulk substrate, such as sapphire, SiC, Si or GaAs, is commonly used in a gallium nitride semiconductor device as a substitute for a GaN bulk single crystal, which is difficult to form. Organic metal chemical vapor deposition (MOCVD), or halide vapor phase epitaxy (halide VPE), has been relied upon for the epitaxial growth of GaN on the principal surface of a bulk substrate. The (0001) C face of Al.sub.2 O.sub.3 (sapphire) is widely used as a bulk substrate. Because a lattice mismatching as large as 13.8% exists between sapphire and GaN, a misfit dislocation is likely to occur from the stress acting upon the crystal lattice during the growth of GaN, resulting in the formation of a dislocation having a high density in the order of 10.sup.8 to 10.sup.10 cm.sup.-2 between the sapphire substrate and a grown GaN layer and also resulting in the failure of GaN to grow into a layer of high quality. This dislocation propagates in the direction of growth of GaN. In a GaN semiconductor laser device that is formed on the principal surface of the substrate, the dislocation extends through the main structure of the element such as the active layer or the layer overlying it, and thereby lowers the characteristics of the laser and its reliability. The propagation or increase of the dislocation is also caused by an electric current applied for driving the device, and causes serious problems including the shortening of its life and the lowering of its reliability. In order to make a GaN semiconductor laser device of improved reliability, it is therefore necessary to form a crystal of improved quality by lowering the density of the dislocation formed between the substrate and the GaN layer, which is as high as 10.sup.8 to 10.sup.10 cm.sup.-2, or by reducing the dislocation from propagating to the active layer of the laser. The same problems occur with the use of any other bulk substrate causing a lattice mismatching with GaN, such as SiC, Si or GaAs.
Attempts have recently been made to form a striped mask of SiO.sub.2 on the surface of a GaN layer grown on a sapphire substrate and to grow GaN again on the mask to form a GaN layer having a low dislocation density. This is known as an ELOG (epitaxially lateral overgrowth GaN substrate) [see (1) A. Usui, et al.: J. Jpn. Appl. Phys., vol. 36, No. 78, pp. L899-L902 (1997), and (2) S. Nakamura, et al.: Appl. Phys. Lett. 72, 211 (1998)]. It is, however, necessary to form a GaN layer having a thickness of at least several tens of microns to cover the SiO.sub.2 mask completely. Moreover, voids are likely to appear above the SiO.sub.2 mask, and it is necessary to form a GaN layer having a thickness of nearly 100 microns to fill the voids and form a perfectly flat surface. An undesirably large amount of material is, however, required for forming such a layer. Moreover, a difference in the coefficient of thermal expansion between the SiO.sub.2 mask and GaN is likely to cause cracking along the edge of the mask. Thus, ELOG is seriously defective from a productivity standpoint.
Japanese Patent Laid-Open No. 64791/1996 discloses protrusions for controlling crystal growth, but they have amorphous sidewall surfaces, and the single crystal serving as the seed for crystal growth is at the bottom of grooves between the protrusions. Defects appearing on the single crystal surfaces extend in the direction of crystal growth, and gather on the amorphous protrusions. The defects make it difficult to form an operating area on the protrusions.
According to the present invention, such difficulty can be diminished and an operating area can also be formed on the protrusions, as will be explained in detail.